A structure of a conventional typical solar cell is shown in FIG. 3. It has a structure in which, on the side of one main surface of a p-type Si substrate 21 formed of single-crystal or polycrystalline Si having a thickness of the order of 0.25 mm, an emitter layer (n+ layer) 22 into which P or the like is diffused at a depth of 0.1 to 0.5 μm is provided, and an antireflection film 23 formed of Si3N4, SiO2, or the like, for reducing the front surface reflectance and a front surface electrode (light receiving surface electrode) 34 for extracting current are formed thereon; on the other side (the side of a back surface) of the Si substrate, a BSF layer (p+ layer) 25 into which Al or the like is diffused in high concentration is formed, and, on this back surface, a back surface electrode 26 is formed.
When a solar cell of this type is fabricated, the front surface electrode 34 is in general formed by the following printing/firing process because of its easiness, low costs, and the like. That is, as a material of the front surface electrode, a conductive paste mixed with a silver powder is in general used; and, after this conductive paste is applied by screen printing or the like, it is sintered at a high temperature in a firing furnace so as to form the front surface electrode. In this electrode formation method, in general, a conductive paste containing a silver powder, a glass frit, an organic vehicle, and an organic solvent as the main components is used.
The contact resistance between the front surface electrode 34 formed by this method and the Si substrate 21 and the wire resistance of the electrode greatly affect the conversion efficiency of the solar cell, and, in order to achieve high efficiency (low cell series resistance, a high fill factor), the contact resistance and the interconnect resistance of the front surface electrode 34 are required to be sufficiently low.
However, in the conventional electrode structure described above, the glass frit contained in the conductive paste is melted during high-temperature sintering, and part thereof runs down to the interface between the Si substrate and the silver electrode and forms a barrier layer, making it impossible to obtain contact resistance which is low enough to achieve a high-efficiency solar cell.
To address this problem, an attempt has conventionally been made to lower contact resistance by performing heat treatment in an atmosphere of hydrogen, immersion in a diluted hydrofluoric acid solution, or the like, after high-temperature sintering. However, treatment with hydrogen is at a disadvantage in terms of mass productivity and cost, considering factors such as special equipment and a large amount of input energy required to perform such treatment and the difficulty of handling hydrogen. Moreover, since treatment with hydrofluoric acid takes a time of the order of several tens of seconds to several minutes, the glass frit functioning as an adhesive in the silver electrode components is melted, causing a problem of a reduction in adherability and hence detachment of the electrode. This sort of problem would impair the reliability of the solar cell.
On the other hand, it has been known that mixing an oxide such as zinc oxide into a conductive paste makes it possible to lower contact resistance without performing the aforementioned heat treatment in an atmosphere of hydrogen, treatment with hydrofluoric acid, or the like, after high-temperature firing, and to provide a good ohmic contact (for example, see Japanese Patent Application Laid-open (kokai) No. 2001-313400, Japanese Patent Application Laid-open (kokai) No. 2004-235276, and Japanese Patent Application Laid-open (kokai) No. 2005-129660). It is true that the ohmic contact formed by mixing an oxide such as zinc oxide into a conductive paste is improved as the additive amount thereof is increased; however, it tends to hamper sintering of silver when the electrode is fired, leading to an increase in interconnect resistance due to an increase in the specific resistance of the sintered silver product. That is, the problem is that, when the additive amount exceeds a certain level, an improvement of the ohmic contact and the interconnect resistance increasing effect act in an opposing manner, interfering with a significant improvement of the performance of the solar cell as compared with the conventional one. Furthermore, another problem is that its drawback such as a decrease in the adhesive strength between the electrode and a wire caused by the occurrence of so-called silver leaching by which, for soldering to a solder coated ribbon for connection with the solar cell, unsintered silver forms a compound with Sn contained in the solder and disappears in the solder results in a decrease in performance of the solar cell or hinders modularization thereof.